Over the course of time you pond or water feature will collect "sludge" this is made up of old decaying matter, be it fish waste, old leaves or anything else that gets into your pond.

You should ideally clean your pond / water feature once a year, some pond owners use pond vacuum cleaners (DON'T try an ordinary vacuum cleaner or even a wet n dry cleaner they wont work)

If you don't have a pond vacuum cleaner it will have to be the old fashioned method of pump the water out (do save as much of this as possible) and literally remove by hand any old decaying debris. Believe it or not a plastic dustpan and soft brush is very good for this. You can even remove the last of the water this way.

You need to remove all the debris because it will rot and give off toxic gasses to any fish you have, It also gives you the opportunity to split any large pond plants you may have.

If you also now notice your pond liner (what ever it is made from) has green algae growing you can clean this off too, with a slightly stiff brush, use a slow running hose to assist. Never rub too hard as you may damage your liner. If you are using a slow running hose you will from time to rime have to switch your pump on to empty the water. Never leave the pump running all the time as you may burn it out.

When it is all clean you can then put the water you saved back ant top up with rain water or from a hose. if you top up from a hose add a dechlorinator before you put any livestock back.